There are a variety of reasons your network Time Machine setup might start excluding certain machines. Rather than address them each individually, the following is a general troubleshooting approach for the full gamut of issues. Follow these guidelines, and your Time Machine setup should be working again in no time.

First, turn off Time Machine on the misbehaving client machine. With the large, decorative switch now in the “off” position, remove the preferences file at Library/Preferences/com.apple.TimeMachine.plist. If you’re dealing with a corrupt preference file, that should resolve the issue immediately.

Next, remove the Time Machine “cookie” from your network share. These invisible files are named with a period, followed by the MAC address (also called the ethernet id) of the “Ethernet 1″ interface on each client machine (not necessarily the network interface you’re actually using). If the machine has a primary MAC address of a1:b2:c3:d4:e5:f6, then you’re looking for a file called .a1b2c3d4e5f6 at the root of the share. Get rid of it, and any identification issues with your Time Machine client should disappear.

Finally, you’ll need to reintroduce your client machine to the Time Machine server. In order to access your share every hour, Time Machine stores authentication data in a keychain when it’s first configured. Delete all references to your Time Machine server from the “System” keychain (most simply with Keychain Access in the Utility folder). Once no memory of the name or IP of the server remains, you can reconfigure Time Machine to back up to your server again.

These steps won’t solve every Time Machine server problem, but this quick checklist can often get your users backing up again, and get you back to working on larger problems.

Recommended Reading: For more information about common backup problems, take a look at Apple’s own Knowledge Base on troubleshooting Time Machine issues.

To configure your workstations to use server-based Time Machine storage, you’ll have to set up network shares. If you aren’t running AFP, your first step is to turn on file sharing. Open Server Admin and select your server from the left. Click “Settings” from the toolbar, then “Services” below it, and check “AFP” from the list of services and save.

Now choose “File Sharing”, then the “Volumes” view below it, and browse to the location you’re assigning as your Time Machine share. Set the permissions as “Read & Write” to the group you’re backing up (for a small workgroup, that could be all your users, while for larger organizations you’ll likely want to split users up across a number of shares). Click the “Share” button in the upper right, and save again. With your share created, switch to the “Share Points” view, and select “Enable as Time Machine backup destination”, saving one last time.

Using Bonjour, Leopard Server will broadcast the share across your local network, making it available as a Time Machine disk automatically in each machine’s System Preferences. Once the user is logged in to the share, the use of a network volume is transparent to the user. On the management side, each backup is created as an individual sparse disk image, allowing portability and preventing user files from getting intermingled. Since users can delete files from these images, it’s wise to back them up off the server to removable media as well.

While it isn’t the answer for every environment, setting up Time Machine through OS X Server combines many of the best features of local and remote backup methods.

Exclude Files From Time Machine Backup:

While the Time Machine pane in System Preferences has an intuitive interface for excluding paths from backup, doing so for multiple files and folders (or worse still, multiple machines) can be time-consuming. Instead, you can specify files to avoid through the command line, either locally or sent over Apple Remote Desktop. In this example, just replace PATH-ONE and PATH-TWO with as many absolute paths as you have directories or files to exempt.

sudo defaults write /Library/Preferences/com.apple.TimeMachine \
SkipPaths -array-add "PATH-ONE" "PATH-TWO"

Change Regular Time Machine Backup Intervals:

Another Time Machine annoyance is the mandatory hourly scheduling, forcing the machine into heavy disk access even when it’s being used for other read/write intensive operations (like video editing and playback). You can adjust the time between backups, however, in the Terminal or by changing the preferences file with Property List Editor (available in Apple’s free Developer Tools).

If you’re altering Time Machine’s schedule programmatically, type the following:

sudo defaults write /System/Library/LaunchDaemons/\
com.apple.backupd-auto StartInterval -int SECONDS

Replace SECONDS with your desired wait between backups, specified in seconds (that’s 3600 per hour). This has the unwanted side-effect of restricting read permissions on the file, so if you want to edit these preferences again or have them run properly, you’ll have to set it’s permissions back to their original configuration like so:

chmod 644 /System/Library/LaunchDaemons/\
com.apple.backupd-auto

Editing the file with Property List Editor lets you skip the second step entirely, but confines you to manually adjusting the settings on each individual machine. A preferable approach is to push the command out to all your Macs using tools like Apple Remote Desktop, JAMF Casper Suite, or LANrev.

Schedule Time Machine Backups:

If you’d like to run Time Machine only at times you’ve chosen, the command can be incorporated into a legacy crontab, by preparing a scheduled job through launchd, or even by embedding the instructions in an Applescript and choosing “Run Script” from the “Alarm” options in an iCal event. The line you need is:

/System/Library/CoreServices/backupd.bundle/Contents/Resources/\
backupd-helper &

Prevent Unwanted Time Machine Dialogs:

By default, if you haven’t yet set a Time Machine volume, you’ll get asked if each and every disk you attach is a suitable backup destination. If you’re utilizing a different backup scheme, these dialogs can become an annoyance quickly. To put a stop to the them, the command you’re looking for is:

defaults write com.apple.TimeMachine DoNotOfferNewDisksForBackup \
-bool YES

Protect Database Files From Time Machine:

Finally, even Apple acknowledges that Time Machine has issues backing up live databases. The cause (and it’s effects) are still unclear, but it doesn’t hurt to be careful with applications which run a database in the background when closed (such as Microsoft’s Entourage). The problem can be avoided by logging out before scheduled backups occur. The database will safely close, and Time Machine will run as expected.

Recommended Reading: While we were proud to have originally figured out this first Leopard tip on our own, the article was later updated to include Josh Wisenbaker’s clever tip to Stop Time Machine Nagging, via the equally clever AFP548.com.